Update to NeoForge 1.20.4

[JulianVennen]

This PR replaces Forge with NeoForge and updates to 1.20.4

• The MixinGradle plugin has been replaced with the NeoGradle mixin plugin
• Librarian is no longer required as NeoForge natively supports parchment
• DistExecutor is deprecated and has been replaced with FMLEnvironment.dist
• FMLJavaModLoadingContext is deprecated, the mod event bus can now be obtained from a constructor parameter
• Oculus and Rubidium don't support 1.20.4 yet, so their dependencies and parts of their code have been commented out

[PepperCode1]

0.6 is discontinued and will not receive any more updates. Superseded by #238.